The present invention relates generally to the manufacture of motor vehicle brake drums and, in particular, to composite drums having a balancing skirt.
Virtually all wheeled vehicles are provided with a brake system for selectively inhibiting the rotation of the wheels and, therefore, slowing the movement of the vehicle. To accomplish this, a typical vehicle brake system includes a friction brake assembly which is provided at one or more of the vehicle wheels. To actuate the friction brake assembly, a driver manually moves the brake pedal of the vehicle. The associated pneumatic or hydraulic actuating system, activates the friction brake assemblies to inhibit the rotation of the vehicle wheel.
Such vehicle friction brake assemblies are generally classified into two types, namely, drum brake assemblies and disc brake assemblies. A typical drum brake assembly includes a hollow cylindrical drum secured to the wheel of the vehicle for rotation therewith, and a brake shoe assembly secured to the nonrotatable components of the vehicle. The brake shoe assembly includes a pair of arced friction shoes which are operatively connected to a pneumatically or hydraulically actuated piston. The friction shoes are disposed within the hollow drum adjacent to an inner cylindrical braking surface thereof. The friction shoes are normally spaced apart from the braking surface of the drum. When the driver of the vehicle manually moves the brake pedal, the piston is actuated to move the friction shoes apart from one another into frictional engagement with the braking surface of the drum. As a result, rotation of the drum and its associated wheel are inhibited thereby slowing the vehicle.
In the past, drums of the type described above have been formed from gray iron using a conventional "as-cast" method. The "as-cast" method simply involved casting molten gray iron into the desired shape of the drum and subsequently cooling, followed by cleaning and machining when necessary. Thus, the "as-cast" method has been found to be desirable because it is a relatively simple and inexpensive method to perform. Also, gray iron has been found to be an acceptable material to use in the "as-cast" method because it provides the resultant drums with sufficient mechanical and physical properties for use in the friction-brake assemblies, such as hardness, strength, wear resistance, thermal conductivity, and the like.
Motor vehicle brake drums are often formed entirely of cast gray iron. However, while cast gray iron is the preferred material for the braking surface of the brake drum, a full cast brake drum is relatively heavy. Fully cast iron brake drums are also brittle, which can lead to heat check crack-through and breakage.
The structure and method of manufacture of such composite brake drums are known. In accordance with the conventional practice, these composite brake drums are manufactured by first forming an annular band from sheet steel stock which is then rolled to the desired configuration, typically having a radially inwardly-turned end flange and a plurality of outwardly directed strengthening ribs. In addition to providing strength, the strengthening ribs also increase the surface area to dissipate heat and improve the cooling capabilities of the brake drum.
With the annular steel band externally fixed, molten gray iron is centrifugally cast therein to form a metallurgically bonded composite brake ring. After the ring has cooled, a steel drum back is generally welded to the flange. The radially inwardly directed braking surface of the ring is then finish machined.
As part of finish machining a brake drum, the drum is balanced. This is commonly known as a "final balance." This is one of the last steps in producing the brake drum. As part of this step, the brake drum is checked for eccentric portions which will cause a brake drum to wobble during rotation. This is also known as a brake drum that is out of around or not "true."
In the case of a gray cast iron brake drum, a static balancer may be used to determine the location and amount of any eccentric portions of the brake drum. If any eccentric portion is noted, this portion may be machined to remove any eccentricity from the brake drum. This was accomplished by removing material from an outer surface of the brake drum. Known techniques have been used to remove the portion or portions from the outer surface of the brake drum.
In the case of a composite brake drum with an annular band it is not desirable to remove material from the outer surface. The machining of any outer surface of a composite brake drum with an annular band would result in removing portions of the annular band. Removing the portions of the band is counter intuitive to the benefits of increased strength and reduced weight gained by using an annular band. It is also not desirable to remove material from an inner surface because the inner surface is the braking surface.